Projection exposure machines for semiconductor lithography are used for the photolithographic production of semiconductor components and other finely structured devices. In this case, the pattern of a mask (reticle) is projected onto a substrate (wafer) usually coated with a photosensitive layer. To this end, a projection exposure machine includes a projection objective with a plurality of optical elements such as lenses, mirrors, gratings or plane parallel plates, which are supported via mounts in an objective housing of the projection objective. The optical elements refract and/or reflect electromagnetic radiation emitted by an illuminating device.
Mounting technology for optical elements of a projection objective is disclosed, for example, in U.S. Pat. No. 6,229,657 B1. U.S. Pat. No. 6,229,657 B1 discloses a mount including an inner ring permanently connected to the optical element, and an outer ring, the inner ring and the outer ring being interconnected via joints, in particular via solid joints, distributed on the periphery.
So-called manipulators can be used to compensate for aberrations. Such manipulators include an optical element and adjusting elements which are distributed on the circumference of the optical element. The adjusting elements can be used to position and/or deform the optical element. These manipulations of the optical element cause a change in the imaging which compensates an aberration of the entire system. In some embodiments, mounts for such manipulators include an inner ring or holder with which the optical element is accommodated, and an outer ring or a support device, it being possible to move the inner ring relative to the outer ring using the adjusting elements.
The joints and/or the adjusting elements decouple the optical elements, preferably from external structures, in terms of deformation. The connections between the optical element and the external structures preferably have a low stiffness.
In addition, imaging of ever smaller structures is desired in microtechnology. Larger and therefore heavier optical elements can be used for this. Both the low stiffness and the greater mass of the optical elements respectively lower the natural vibration frequencies of the system. This increases the risk of exciting the optical elements to vibrations from outside.
It is therefore disclosed, for example, in WO 2006/084657 A1, to provide a damper element between the outer ring and the inner ring.
Furthermore, it is disclosed in WO 2007/006577 A1 to provide a mass damper with an additional mass or a plurality of additional masses on the optical element, vibrational energy of the optical element being dissipated by friction using the additional mass(es). Such an additional mass is denoted below as damper mass. WO 2007/006577 A1 discloses fitting an annular damper mass on an optical element or an inner ring using a damper element. The damper element is arranged between the annular damper mass and the optical element or the inner ring and is, for example, a fibrous material such as paper, felt or an elastomer. Such mass dampers are also denoted as annular absorbers. A good vibration damping can be attained by means of annular absorbers for systems with one degree of freedom.